Download Magnets Review

Magnetism
What is a magnet?
• A magnet is a material or object that produces
a magnetic field.
• Question: What is a magnetic field?
– A force that pulls on certain materials, and attracts
or repels other magnets
– Is created by the rotation of electrons around the
nucleus of each atom
Classification of Materials
1. Magnetic
– A material which is strongly attracted to iron, nickel
or cobalt.
– It also repels or attracts another magnets.
2. Ferromagnetic
– Is attracted to a magnet.
– Becomes magnetic temporarily, (is only a magnet as
long as in the presence of a magnet).
– Becomes demagnetized if tapped.
3. Non-magnetic
– Not attracted by a magnet at all.
Magnetic Theory
• Only magnets and ferromagnetic substances
are affected by magnetic fields.
• In these materials, small groups of atoms
band together in areas called domains.
– The electrons of the atoms in a domain are all in
the same magnetic orientation.
• The electrons are all oriented in the same way!
Magnetic Theory
• In a non-magnetized ferromagnetic substance,
these domains are in a random order.
• When these substances are placed in a magnetic
field, their domains will align in the same
direction as the field, and become magnetized!
– The moment the magnetic field is removed,
ferromagnetic substances remain magnetic for only a
short while, and their domains loose their specific
arrangement.
• Note - the domains in magnets are always
aligned!
Magnetic Theory
What’s the shape of a magnetic field
Bar Magnet
What’s the shape of a magnetic field
Horseshoe Magnet
Magnetic Fields
• All magnetic fields have the following
characteristics:
– A north and a south pole.
– The lines of force goes from one pole to the other
pole
• They start from the north and end at the south.
– The lines of force never intersect!!!
– Like poles repel one another, whereas opposite
poles attract one another.
Law of Magnetism
• Like poles repel one another.
• Two north poles will repel one another! None of
the lines of force will go near the pole. The lines of
force will go towards their own south poles.
• Opposite poles attract one another.
– A north pole will attract with a south pole. The
lines of force will start from the north pole and
head to the south pole.
Attraction between Magnetic Poles
Repulsion between Magnetic Poles
How compasses react to magnets
• The red end of a compass needle is attracted
to the south pole of a magnet.
– Therefore, the red end of a compass is that
compass’s north pole!
• Question: why does the red end of a compass
needle point towards the North Pole where
Santa lives?
– The Geographical North Pole is a MAGNETIC
SOUTH POLE!
Magnetic Remanence
Definition: Ability of a material to acquire and
keep its magnetic properties.
Two types of magnets:
• Temporary → they loose their magnetic
ability quickly.
• Permanent → they stay magnetic for a long
time.
Magnetic Remanence
• So, materials that stay magnetic for a long
time have high magnetic remanence.
• Same applies to materials that don’t stay
magnetic for a long time. Materials that are
only magnetic for a short time have low
magnetic remanence.
Electromagnetism
• In short, joining of electricity and magnetism!
Remember magnetism has to do with the alignment
of the electrons in atoms.
• When you have an electronic current, the motion of
electrons are going in the same direction.
• Therefore, my electrons have the same orientation.
• When you have a dynamic current flowing, you
produce a temporary magnetic field.
– No current = no more field
Right Hand Rule!
• Used to determine the direction of the magnetic
field lines generated by a live wire
• Your thumb: points in conventional current direction
– Tip of thumb points to (-) terminal. Butt of hand is pointing
to (+) terminal
• Your fingers: show direction of magnetic field line
– The curling of your fingers grabbing the “wire” curl in the
direction the wire is going
Right Hand Rule!